Polyester films have been extensively used as a base material for a film in various application fields such as building materials, optical materials, wrapping materials, etc., because they are excellent in mechanical properties, dimensional stability, heat resistance, transparency, electrical isolation properties, chemical resistance, etc.
However, the polyester films have a low surface hardness and a low abrasion resistance. Therefore, there tends to arise such a problem that the polyester films suffer from flaws during various processing steps or during use thereof.
For the above reason, in order to enhance the surface hardness and the abrasion resistance of the polyester film, it is known that a hard coat layer is formed on a surface of the polyester film (Patent Literature 1). However, the hard coat layer generally has low adhesion properties to the polyester film, and therefore tends to be peeled off from the polyester film during various steps or during use thereof.
In order to solve the above problems and enhance the adhesion properties to the polyester film, there is known a method in which an easy-adhesion layer using a binder such as acrylic resins, urethane resins, polyester resins, etc., is formed on a surface of the polyester film (Patent Literature 2), and a method in which the polyester film is subjected to surface activation treatment such as corona treatment. In addition, it is also known that polyfunctional acrylates and melamine compounds are used in the hard coat layer to thereby enhance the adhesion properties to the polyester film without conducting the above-described treatment (Patent Literature 3). In this method, it is possible to use an in-line coating method in which formation of the polyester film and formation of the coating layer on the polyester film are simultaneously conducted. This method, therefore, is more excellent in production cost than the above-described methods of enhancing the adhesion properties. However, in the above-described methods, the coating layer tends to hardly exhibit satisfactory adhesion properties to the polyester film.